1. Field of the Invention
The invention relates to a thermal-type infra-red ray solid-state image sensor, and a method of fabricating the same, and more particularly to a thermal-type infra-red ray solid-state image sensor including a hood for enhancing an aperture ratio of a pixel, and a method of fabricating the same.
2. Description of the Related Art
Japanese Patent Application Publication No. 2001-215151 has suggested a thermal-type infra-red ray solid-state image sensor which is capable of allowing a thermal-type infra-red ray detector to have higher sensitivity and enhancing an aperture ratio.
FIG. 1 is a cross-sectional view along a current path in a unit pixel in a thermal-type infra-red ray solid-state image sensor suggested in the above-mentioned Publication.
The illustrated solid-state image sensor is comprised of a silicon integrated circuit substrate 1 in which a signal-readout circuit 24 is fabricated, a reflection film 2 composed of metal and formed on the substrate 1, a first electrically insulating protection film 18 formed on the substrate 1 to cover the reflection film 2 therewith, a infra-red ray receiver 19 formed above the first electrically insulating protection film 18, and a pair of supports 6 supporting the infra-red ray receiver 19 such that the infra-red ray receiver 19 floats above the first electrically insulating protection film 18 with a cavity 20 therebetween.
The infra-red ray receiver 19 is in the form of a diaphragm (accordingly, the infra-red ray receiver 19 is often called “a diaphragm”), and is arranged in each of pixels.
The infra-red ray receiver 19 is comprised of a bolometer thin film 11 acting as a temperature detector, two electrodes of a metal wire 13 making electrical contact with the bolometer thin film 11, and electrically insulating protection films 21, 22 and 23 surrounding the bolometer thin film 11 and the two electrodes.
The support 6 is comprised of the electrically insulating protection films 21, 22 and 23, and includes a beam 6a extending in parallel with a surface of the substrate 1, and a leg 6b connected to one of ends of the beam 6a. The electrically insulating protection films 21, 22 and 23 defining the support 6 surrounds the metal wire 13. Though the beam 6a may seem quite short in FIG. 1, the beam 6a actually extends along a side of the infra-red ray receiver 19 in order to reduce thermal conductance, and is connected at an end to the infra-red ray receiver 19. The metal wire 13 is electrically connected at one end (that is, an electrode) to the bolometer thin film 11, and at the other end (that is, the other electrode) to a contact electrode 3 of the signal-readout circuit 24.
A hood 10 extends from the infra-red ray receiver 19 so as to cover the electrodes of the infra-red ray receiver 19, the support 6 and the contact electrode 3 therewith with a space therebetween.
When infra-red ray is irradiated to the electrically insulating protection films 21, 22 and 23 and the hood 10, a part of the infra-red ray is absorbed into the electrically insulating protection films 21, 22 and 23 and the hood 10, and resultingly, the electrically insulating protection films 21, 22 and 23 and the hood 10 are heated. The rest of the infra-red ray passes through the infra-red ray receiver 19, the hood 10 and the support 6, and goes towards the substrate 1. Then, the infra-red ray having passed through the infra-red ray receiver 19, the hood 10 and the support 6 is reflected at the reflection film 2, the metal wire 13 and the contact electrode 3 towards the infra-red ray receiver 19 and the hood 10, and thus, enters the electrically insulating protection films 21, 22 and 23 and the hood 10 again. As a result, the electrically insulating protection films 21, 22 and 23 and the hood 10 are further heated.
Heat generated in the hood 10 is transferred to the bolometer thin film 11 through the electrically insulating protection films 21, 22 and 23. As a result of heat transfer to the bolometer thin film 11 from the electrically insulating protection films 21, 22 and 23 and the hood 10, a temperature of the bolometer thin film 11 varies, and hence, a resistance of the bolometer thin film 11 varies. The signal-readout circuit 24 of the substrate 1 converts the variance of a resistance of the bolometer thin film 11 into variance of a voltage. An external circuit reads out the thus converted voltage as an electric signal, and makes infra-red images based on the read-out voltage.
In the thermal-type infra-red ray solid-state image sensor illustrated in FIG. 1, since the hood 10 extending from the infra-red ray receiver 19 covers the electrodes of the infra-red ray receiver 19, the support 6 and the contact electrode 3 therewith with a space therebetween, it is possible for each of pixels to have a higher aperture ratio and absorb much infra-red ray, ensuring higher sensitivity.
In the thermal-type infra-red ray solid-state image sensor illustrated in FIG. 1, the infra-red ray receiver 19, the support 6 and the hood 10 are all comprised of a silicon nitride film, a silicon oxide film or a silicon oxynitride film. The infra-red ray receiver 19 and the support 6 are comprised of an electrically insulating protection film in a common layer. To the contrary, since the hood 10 is designed to extend to cover the electrodes of the infra-red ray receiver 19, the support 6 and the contact electrode 3 of the substrate 1 therewith with a space therebetween, the hood 10 is comprised of a film in a different layer from that of the infra-red ray receiver 19 and the support 6. This causes a problem that there is an unnecessary portion of the electrically insulating film of which the hood 10 is comprised (hereinbelow, such a portion is referred to simply as “unnecessary film portion”), existing directly on the infra-red ray receiver 19, and not contributing to enhancement of an aperture ratio. If the unnecessary film portion remains as it is without being removed, there would be caused unnecessary increase in a thermal mass in the infra-red ray receiver 19 with the result of deterioration in thermal response performance of the infra-red ray receiver 19.
In order to such deterioration in thermal response performance of the infra-red ray receiver 19, it is necessary to remove the unnecessary film portion by etching. Hence, the unnecessary film portion is removed in the thermal-type infra-red ray solid-state image sensor illustrated in FIG. 1 (the unnecessary film portion is not illustrated on the infra-red ray receiver 19 in FIG. 1). According to the above-mentioned Publication, in a step of patterning an electrically insulating film into a hood, the unnecessary film portion is etched for removal.
Since it is necessary in the step of patterning an electrically insulating film into a hood to surely separate the electrically insulating film into hoods in each of pixels, the electrically insulating film has to be over-etched such that the electrically insulating film is etched in a depth equal to or greater than a thickness of the electrically insulating film. Accordingly, the thermal-type infra-red ray solid-state image sensor suggested in the above-mentioned Publication is accompanied with a problem that an electrically insulating film of which the infra-red ray receiver 19 is comprised is much etched, and furthermore, it is difficult to control etching of the electrically insulating film, resulting in variance in characteristics among pixel, wafers and/or lots. Furthermore, if the electrically insulating film is too over-etched, the electrically insulating film of which the infra-red ray receiver 19 is comprised is broken with the result that the bolometer thin film is damaged.
In order to avoid such problems, it is necessary to enhance an accuracy with which a step of etching the electrically insulating film of which the hood 10 is comprised is carried out. As another solution, an etching stopper film may be formed at an area where the above-mentioned unnecessary film portion is to be deposited, before depositing the electrically insulating film of which the hood 10 is comprised. However, this solution is accompanied with another problem that additional steps have to be carried out for forming and patterning an etching stopper film, and a term for fabricating a thermal-type infra-red ray solid-state image sensor is unavoidably lengthened.
Japanese Patent Application Publication No. 2002-340684 has suggested a thermal-type infra-red ray solid-state image sensor including a detector, and a signal processor for processing signals transmitted from the detector, both of which are formed on a common silicon substrate. The detector has an electrically insulating film having a thickness smaller than a thickness of an interlayer layer of the signal processor.
Japanese Patent No. 2987198 based on PCT/GB90/01391 (WO91/05284) has suggested a method of fabricating a mechanical micro-switch, including the steps of forming a first sacrifice layer on a substrate, forming an island-shaped second sacrifice layer on the first sacrifice layer, forming a switching device layer on the second sacrifice layer, the switching device layer being composed of resilient material, defining an outline of a switching device on the switching device layer, defining an outline of a window, etching the second sacrifice layer through the window to horizontally undercut the switching device layer, and etching the first sacrifice layer through the etched second sacrifice layer to form a space below the switching device layer.
Japanese Patent Application Publication No. 10-185681 has suggested an thermal-type infra-red ray sensor including an infra-red ray receiver comprised of an absorption layer receiving infra-red ray and converting the received infra-red ray into thermal energy, and a sensor having material values varying in accordance with a magnitude of the thermal energy. The infra-red ray receiver is supported above a semiconductor substrate with a space therebetween by means of a support comprised of a cross-linking portion, a first pillar, and a second pillar. The cross-linking portion, the first pillar, and the second pillar are formed below the infra-red ray receiver, and are partially or entirely covered with the infra-red ray receiver.
Japanese Patent Application Publication No. 2003-106895 has suggested a method of fabricating a thermal-type infra-red ray detector including a pixel in which a diaphragm having a bolometer layer is kept floating by means of a beam fixed at one end thereof to a substrate. The method includes the steps of forming a second sacrifice layer on the diaphragm before removal of a first sacrifice layer, forming a window layer on the second sacrifice layer, infra-red ray being able to pass through the window layer, simultaneously removing the first and second sacrifice layers through a through-hole formed throughout the window layer, and forming a vacuum-sealing layer on the window layer to clog the through-hole after evacuating a cavity resulted from removal of the first and second sacrifice layers.